Screen printing of adhesives is known in the art and is used advantageously to apply adhesives to selected areas on a substrate. The adhesive printed or coated areas can subsequently be used to adhere to a second substrate. Typical screen-printable adhesives are pressure-sensitive adhesives which are tacky at room temperature, or heat-activatable adhesives, which are not tacky at room temperature, but become tacky when heated. Examples of screen-printable adhesives include (meth)acrylic polymers and copolymers dispersed in an organic solvent or water.
Acrylic adhesives, both pressure-sensitive and heat-activatable types, are widely used in industry because they are stable over time, and they can be formulated to adhere to a wide variety of different surfaces. Typical acrylic adhesives are prepared as taught in U.S. Pat. No. 24,906 RE (Ulrich). With the advent of more stringent environmental controls, the technology in adhesives in general has evolved from solvent-based materials to water-based materials, and to a degree, solvent-free materials. Solvent-free acrylate adhesives are known and fall in various categories of processing such as heat-activatable coating and radiation curing which includes E-beam curing, ultraviolet light processing, and gamma radiation processing. Solvent-free crosslinked compositions are known in the art, but they would provide little utility for adhesively bonding to other substrates since they are highly crosslinked and do not flow or become tacky on heating.
Ultraviolet light processed adhesives are described in U.S. Pat. No. 4,181,752 (Martens et al.). While known adhesives processed by ultraviolet light have their own utility and advantages, they do not screen print well because they tend to become stringy during screen printing. Thus, an ongoing need exists for pressure-sensitive and heat-activatable screen printable adhesives that are solvent-free, can be screen printed without the use of a solvent, and provide good shear strength and peel strength.
An adhesive that has the ability to establish multiple discreet electrical connections, often in extremely close proximity, between two substrates is known as an "anisotropically conductive adhesive." Typically, these adhesives are in the form of transfer tapes or free standing films where an insulating adhesive matrix contains sufficient conductive particles to allow electrical conduction through the thickness of the film (the z-axis) while providing no conductivity in the plane of the film. Such film types are known as "z-axis adhesive films" or "ZAF." A typical use for this adhesive is to provide connection between a flexible printed circuit and a rigid circuit such as a flat panel display or epoxy-glass laminate printed circuit board.
Several ZAF materials are described in the literature. Some of these ZAF materials use non-reactive hot-melt type adhesive compositions such as styrene/butadiene/styrene block copolymers. They provide a long shelf life and short bond times at low temperatures. However, they show poor resistance to elevated temperature and humidity aging. Other ZAF materials use thermoset resins that crosslink, usually with the aid of curatives or catalysts, at the bonding temperatures. However, these ZAF materials typically require high bond temperatures, such as 170.degree. C. or higher, and are difficult to use on temperature sensitive substrates.
Additionally, known ZAF materials are manufactured using solvent casting. Solvents usually must be captured or destroyed, and solvents can lead to damage of substrates and components. Additionally, the use of catalysts which are effective at lower temperatures typically leads to reduced shelf life of the ZAF. The use of photoactivated curatives in ZAF materials is also known. However, these adhesives need to be protected from light to avoid premature photoactivation. The methods of the present invention utilize heat curable adhesive films that are capable of rapidly bonding at low temperature, have a long shelf-life at ambient temperature, and provide stable electrical/and or thermal connections over a prolonged period of time.